Broccoli & Pressure: the untold story of the better cooking method

The 2007 study from the Italian Universita’ Degli Studi in Potenza evaluated how much vitamin C and Sulforaphane – the most powerful antioxidant you never heard of – were retained from farm to table, and this is how they did it.

Although I already made a quick reference to this broccoli study in the past, I wanted to revisit it in detail to share more about it. There are lots of interesting tidbits, there, including data that’s been hidden by omission by myself and just about anyone else who has cited this study to promote the nutritional benefits of pressure cooking.

Aren’t you curious about their findings, now?!? Ok, first let’s find out what, exactly, was measured…

hip info: measured antioxidants

Vitamin C – also known as L-ascorbic acid, is a water-soluble vitamin that is naturally present in some foods, added to others, and available as a dietary supplement. Humans, unlike most animals, are unable to produce their own vitamin C and the body needs it to build collagen, metabolize protein to protect against cardiovascular diseases and stimulate the immune system.

Sulforaphane – Sulforaphane is a phenolic phytonutrient with beneficial antioxidative properties. Broccoli and other cruciferous vegetables don’t already contain it. This compound is actually produced when the vegetable is damaged by either chewing, addition of liquid or processing. There’s is evidence of possible benefits for the treatment of osteoarthritis, certain cancers, respiratory illnesses and skin and blood disorders.

the method

The study tracked the antioxidant content of broccoli from harvest, through storage (refrigerator and freezer) and then after five different cooking methods (boiling, steaming, microwaving, pressure cooking and microwave pressure cooking).

The broccoli was trimmed, washed and transported to the lab via a refrigerated van within two hours of harvest. The experiments and measurements began immediately after the vegetables arrived at the lab.

Nordicware Tender Cooker (Microwaveable pressure Cooker)

The refrigerated (chilled) broccoli was stored in a climatic chamber to simulate refrigerator temperatures. The frozen broccoli was steam-blanched blanched, cooled down in water, drained and dried prior to being packed in freezer bags and stored in a deep chest domestic freezer.

When boiled, broccoli was added to boiling tap water in a ratio of1:8 by weight (approx 750g of broccoli in 6L of water) and then cooked for 15 min. The steamed broccoli was cooked for 23 min with a broccoli/tap water ratio of 1:2. The pressure cooked broccoli was treated for 2 min (broccoli/tap water 2:1 b.w.). When the broccoli was microwaved it was zapped for 11 min at 600 W using a domestic microwave oven (Panasonic) with the broccoli/tap water ratio of 6:1. And when they used the microwave pressure cooker for the broccoli it was zapped 7 min using a 14:1 broccoli/water ratio using a Nordicware microwaveable pressure cooker.

the findings

Get fresh broccoli and cook ’em right away!

While the broccoli used in this study were harvested just two hours prior to the testing, broccoli you find in the supermarket have been picked, shipped to a reseller, then to a distribution center, then to the supermarket and sat on a shelf waiting for you to come and buy them. So, they could have been picked days, maybe even a weeks before you actually bring them home. Even if you’re lucky enough to buy your broccoli at the farmer’s market their Vitamin C will slowly deteriorate while sitting in the fridge. The exact amount of nutrient loss will vary between cultivars and time of harvest, according to the authors who compared their results with similar research.

In this study, the vitamin C decreased by 18% after a week in the fridge, and an additional 20% after three weeks – with a total vitamin loss of 38% compared to fresh broccoli!

Sulforaphane concentration went downhill during storage, too, with a recorded final loss of 29% after four weeks of storage.

Freezing broccoli stops nutrient loss but…

The pre-freeze blanching was to blame for a 32% Vitamin C loss. Once the broccoli was frozen, the nutritional properties did not decrease any further for up to two months – which is the last measure for this study. The nutrient reduction is actually similar to what was measured in steamed broccoli (details further down) – even though that broccoli was steamed for a longer amount of time.

Pressure cooker wins, but microwave pressure cooker wins more

In the past, when sharing results from this study, for myself and others, the focus has always been on how pressure cooking shines compared to conventional cooking methods – but the real untold story, here, is how microwaveable pressure cookers perform.

The researchers concluded that both the pressure cooker and microwaveable pressure cooker are good tools for cooking broccoli:

In particular, boiling and steaming caused vitamin C losses of 34.2% and 22.4%, respectively, which were significantly different from those observed for all the other cooking methods (P < 0.05). Conversely, the other heat treatments (pressure, microwave, and pressure/microwave cooking) did not cause any significant loss of vitamin C in comparison with the fresh reference sample.

In the present research, microwave cooking determined a 39% reduction of sulforaphane in broccoli, similarly to what reported by Howard and others (1997) in an analogous study. Pressure cooking determined a 16.8% reduction of sulforaphane, while pressure combined with microwave cooking did not cause any deleterious effect (P < 0.05) (Table 1 [excerpted above] ).

So, even though the pressure cooker beats steaming, boiling and microwaving in nutrient retention – the real shocker is that using a microwave pressure cooker the researchers saw a previously unheard-of nearly complete retention of Vitamin C and Sulforaphane compared to fresh broccoli.

I’ve never personally tried a microwaveable pressure cooker (I don’t have a microwave) but, at least for broccoli, they make a compelling case for nutrient preservation.

Do you have a microwaveable pressure cooker? Comment, below.

now, pressure cook that broccoli!

If you have a microwaveable pressure cooker, you can make hip recipes, too. If a browning step is called-for simply do it in another pan before transferring the food into the microwaveable pressure cooker and then cook for the time recommended by the manufacturer of the microwaveable pressure cooker.

Now you mention it, so do I. I steam broccoli for about 6 minutes too. I steam for 5, poke the end and steam another minute if it is still hard.

I also find the water ratios strange. 1:2 for steaming? You just need enough water so the pot doesn’t boil dry in 7 minutes. (Or 24 for their mush!) I admit I haven’t ever measured the ratio, but it seems very high. Not that it should matter as you are cooking in the steam not the water.

If they used George’s mum’s recipe, no wonder George W. didn’t like his broccoli.

I have never pressure cooked broccoli either as I am happy with steaming, but the 2 minutes cited here is at odds with Laura’s “3-5 minutes”. Which coupled with her “normal” release would take longer than 6 minutes steaming. Which is probably why I have never bothered.

“steam for 5, poke the end and steam another minute if it is still hard”

Yup, same for me. If the ends of the florets are large / thick I’ll split them with a knife. The split part opens up in the steam and cooks quicker. Then the florets don’t go mushy waiting for the ends to soften up.

I went to the trouble & expense of downloading the PDF of the research paper before commenting. Here’s what I found:

1) Their produce-to-water ratios are given BY WEIGHT, not by volume as most household recipes are given (note that in her writeup above, Laura correctly indicated that the ratios are given “by weight”). So if in your head you are thinking 1 cup of broccoli to 8 measuring cups of water, you are already thinking wrong, and your understanding of the methods and results presented will be equally wrong. “By weight” (sometimes abbreviated as “b.w.” or “bw” after the values are given in scientific literature) is the most accurate experimental method for solids & liquids, whereas common household “by volume” recipes are easier for most people but definitely not good enough for scientific evaluation.

2) Here is an excerpt from the actual research paper explaining their approach to cooking times for the various methods:

“In order to replicate ordinary cooking conditions, the cooking settings (time, temperature, broccoli/water ratio) were chosen according to what reported in traditional recipes or suggested in the cooking booklets provided by the manufacturers of the kitchenware. Slight adjustments were necessary in order to obtain broccoli having similar characteristics, and the best combinations among the different operating parameters were identified on the basis of preliminary sensory tests. Appearance and texture were evaluated by using a panel consisting of 5 people trained according to international standards (ISO2005), and in their training a reference sample of fresh broccoli was used. In particular, the panelists evaluated texture (chewing resistance and crispness) and appearance (greenness and brownness) by quantitative descriptive analysis, using unipolar continuous line scale (ISO2003).”

So it seems to me, having actually read the original research paper, that the researchers probably were not as crazy as most of the commenters here might think, rather it would appear that they took the palatability of the resulting cooked broccoli into account and adjusted their cooking times for the best uniformity that is reasonably achievable. NOTHING is ever perfect, nor perfectly uniform, but the point is, the researchers DID consider the uniformity of the cooked product in their research.

Interestingly, at other places in the research paper they do mention that for microwave cooking, multiple studies have indicated that the power levels and cook times when microwaving was a significant factor in the resulting nutrient retention. It might pay to dig for more research papers on that topic alone.

Anyhow, this is a very interesting topic, more complex than most of us wish to admit and/or contemplate, but intuitively it makes sense to me that the best nutrient retention would appear to result from microwave-pressure-cooking. Lower time-at-temperature in a microwave-pressure-cooker than in a conventional pressure-cooker, and it’s all captured in the sealed container, with the least volume of water into which to leach the nutrients. So for me, that’s the takeaway – conventional boiling will always be the least desirable, steaming is better but not by a lot, microwaving is decent, conventional pressure cooking is good, and apparently microwave-pressure-cooking is the best (at least for retention of the two specific nutrients tested, specifically when cooking broccoli ;-)

WPBF, good on you for downloading and reading the paper yourself! Your comment actually clarified one thing for me about the taste tests. I only saw them mentioned at the beginning of the study and it left me wondering…. so, how did they do? But now I realize that the tests were used to determine the cooking time. I somehow missed that because I was so focused on finding the results.

I feel obliged to point out that there are cultural preferences on the “doneness” of vegetables. I lived in the United States for almost 25 years and when I first moved back to Italy I was really turned off by the gray-cast overly-tender vegetables I was served. Now, when I visit America in the summer I can’t believe the plates of bearly-cooked crunchy vegetables I’m served at restaurants.

Since this study was conducted in Italy, where there is the expectation that a vegetable be tender enough to easily slice with the side of a fork, I’m pretty sure that the results would not be palatable to a majority of my America- and Aussie-based readers. ; )

Gotta agree with the others, when I nuke my bite-size pieces of broccoli, I do not add any water, a small pat of butter, a little salt and it’s only in there for 1.5 minutes. Eleven minutes would result in mushy, inedible broccoli. And the 15 and 23 minute boil/steam? Again, inedible. The study is interesting, but no one found the inedible end product unacceptable?

Hi Laura I truly love your broccoli soup so I decided to make a side dish with the broccoli and potatoes.
I added 1 1/2 cups of water to the pressure cooker and placed the steamer basket in the pot then I quarted about 6 potatoes (the potatoes where not to big not too small so the quarters where just the right size) and placed them on the steamer basket. I then inserted the flowerett stems into the potatoes and cut the stem of the broccoli in half and inserted them also. I then closed the lid and cooked them for 5 mins. on high and did a quick release. Then with just a little seasoning to taste what a great side dish.

Basically the same recipe with a few things taken out so it can be used as a side dish. Thanks again for such a great web site.

I have a microwave pressure cooker. My brand is Nordic ware and it is very user friendly.
The nice thing is that there is no wait time for cooking. You do not have to wait for pressure to build up.
You just dump in the ingredients, put on the lid and place it in the microwave and set the time.
Foods come out great. The only thing is it is small for a family but it would work great for a big family to cook vegetables.

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